1. Field of the Invention
The present invention relates to ultrasonic imaging transducer arrays designed for medical applications, and, more particularly, to transducer arrays based on multilayer piezoelectric structures which provide improvements in the electrical and acoustic behavior of the transducer arrays and to methods of making such arrays.
2. Description of the Related Art
Imaging human organs by ultrasound is an essential modality in most medical specialties and particularly in the fields of obstetrics, radiology and cardiology. The ultrasound or ultrasonic transducer is the limiting element that determines the quality factor of the diagnostic imaging. Several types of ultrasound transducers exist, ranging from sector moving single elements, to electronic linear arrays and to multidimensional arrays. The latter is the most sophisticated device currently available for imaging applications.
Conventionally, linear arrays comprise elementary transducers arranged along a single axis, while multidimensional devices comprise elements disposed in orthogonal planes to provide either am expanding lateral focus or crossing B-mode planes useful for rendering 3D images. Additionally, the term xe2x80x9c1.5Dxe2x80x9d array is given to a linear array having transducer rows independently addressable in elevation. This arrangement provides the possibility of producing a better focussed ultrasound beam in the depth direction of examination by switching and synthesizing apertures along this direction. Similarly, the term 2D array or matrix array is usually employed for transducers having elements of square shape uniformly distributed in two orthogonal directions of the front plane. This arrangement permits the transducer device to perfectly synthesize the beam pattern or to correct phase aberrations due to the tissue or acoustic aperture when used in a conventional application. Volumetric or multiplane imaging approaches are only feasible with this type of device if movement of the transducer device during the image acquiring process is not desired.
Geometric specifications for linear phased arrays require that each elementary transducer exhibit an angular response of +/xe2x88x9245xc2x0 which results in an acoustic aperture of about one wavelength for each individual transducer. This requirement enables the array to limit the emergence of grating and side lobes that introduce artifacts into the image. This requirement must be observed for any imaging array device including those designed for 1.5 and 2D use. Because the frequencies of transducer are usually in the range of 3 to 10 MHz, the elementary transducer aperture inherently varies from 0.150 mm to 0.50 mm in width in order to satisfy the acoustic radiation requirements. These dimensions pose severe fabrication difficulties in achieving repeatable elements and interconnections.
Other difficulties that affect the design of such narrow band transducers concern the surface of the transducer element forming the device and the mismatch in electrical impedance created when compared to conventional electric circuits. Inherently, low sensitivity and an oscillating or undamped impulse response can be observed with such transducer elements. This drawback is partially overcome in a linear phased array by increasing the elevation aperture of the array in order to expand the surface area of the transducer element. Unfortunately, this solution is not suitable for 1.5D and 2D array devices where the surface area of each element is completely bounded or predetermined. In this regard, the worst case is that of the 2D array where square shaped element transducers are required.
Considering this point further, as discussed above, considering this point further, a critical problem associated with 2D array construction concerns the electrical and capacitive characteristics of the transducer elements. In this regard, it is noted that the capacitance of a piezoelectric or dielectric element is a function of the dielectric constant, the surface area and the thickness of the material. Specifically, the capacitance is calculated based on the relation:       C    =                            ϵ          r                xc3x97        S            e        ,
where xcex5r represents the relative dielectric constant of the piezoelectric material, S the surface area and e the thickness of element. The capacitance of a transducer element directly governs reflection of the incident energy and this effect is emphasized when the capacitance value is substantially lower than that of the transmission line. Furthermore, a lower transducer capacitance also tends to lower the energy storage capability of the transducer device so that sensitivity is significantly affected.
The recent development of high dielectric constant piezoelectric materials such as relaxor based ceramics or single crystals has resulted in high density linear phased arrays which outperform conventional transducers made of standard ceramics. These new transducer constructions employ piezoelectric materials having a relative dielectric constant as high as 5000 in order to minimize capacitance loss. However, this approach to optimizing transducer devices is only obtained at the expense of a severe limitation on the operating temperature to avoid risk of depolarization or premature aging of the device.
Returning to a consideration of transducer arrays of 1.5D and 2D configurations, the excessively small surface area of the transducer element undermines the advantages associated with the materials described above, so that difficulties are encountered by engineers in the development and manufacture of such devices. In order to overcome the capacitance mismatch problem, attempts have been made to integrate active impedance matching into the device and to provide built-in driving circuitry connected to each transducer, with some relative success. However, the heating of such components results in a rapid rise in the temperature of the transducer and therefore results in excess current regulation for medical devices. Other attempts involve the use of multilayer structures in 2D arrays wherein sophisticated manufacturing techniques have been implemented, such as micro via methods and screen printing processes. However, such a fabrication process is not suitable, in practice, for low volume production and thus this technology for transducer fabrication still remains in the laboratory prototype stage. Further, the acoustic performance is yet to be confirmed.
Turning now to 1.5D arrays wherein the obstacles encountered are less important than for 2D arrays, the main problem regarding this kind of transducer is the variation in the surface of the transducer element when viewed in a common elevation plane. The transducer elements, which are conventionally disposed in the azimuth direction are further arranged in parallel rows that are organized in concentric manner along the elevation plane. Indeed, such rows have the elevation dimension thereof shifted in a manner so as to exhibit a wider aperture at the central area of the transducer array and provide the narrowest aperture at the edge of transducer.
In order to optimize the geometric aperture of a 1.5D array, a Fresnel synthetic aperture may be advantageously provided in the elevation plane. However, this approach presupposes that the transducer is equipped with row apertures downshifted or varied according to a specific law of progression designed to reduce the side lobes emanating from such a synthetic aperture construction. Advantages relating to the provision of an elevation synthetic aperture for an imaging linear array include the ability to modify the focal distance in the plane of space in contrast to conventional devices which only provide a fixed focus. However, in the construction of such devices a major obstacle is encountered which has not been fully addressed in the prior art. This obstacle concerns the electrical impedance variation between a transducer element belonging to a given row and that of another row. This varying impedance characteristic leads unavoidably to a dramatic decrease in the sensitivity and bandwidth of apertures having a smaller elevation dimension, thus producing focus aberration. Reported systems have attempted to overcome this problem by implementing a predetermined gain compensation for the corresponding driving circuitry, but this approach impacts on the price of the apparatus and encumbers the construction thereof because these add-on circuits must be located on the vicinity of the transducers in order to be efficient.
With regard to 2D arrays, there have been reported at least two principal construction technologies, viz., collective on-chip construction and conventional acoustic construction. Each of these technologies suffers specific advantages and weaknesses which are summarized below.
Conventional acoustic transducer construction is perhaps the most elementary technique employed for obtaining 2D arrays, in that each transducer element of the array is individually considered and built, and the resultant assembly of a plurality of transducer elements forms the array. The approach is based on the provision of a piezoelectric member mounted on a backing block which serves to maintain the geometry of array and to provide elimination of backward acoustic reflections. The front face of the devices is commonly loaded by matching layers so as to optimize energy propagation in the medium under investigation. This technology offers interesting transducer performance in linear array constructions but has proven to be subject to very difficult fabrication problems when applied to 2D arrays because of problems associated with making electrical connections and the overall complexity of assembly of the construction.
Another technique of forming 2D transducer arrays concerns the use of piezoelectric elements performed on silicon or insulated substrates. Such a backward insulated substrate facilitates providing electrical connections for the transducer elements and this lowers the fabrication costs. However, the high acoustic impedance of such substrates and the low acoustic attenuation coefficient of the material used make the resultant device subject to strong artifacts from echo signals due to reverberations in the thickness of the substrate. Regarding the large number of elements to be connected in 2D arrays, although this results in certain weaknesses in acoustic response, investigations into collective fabrication methods for obtaining 2D array have been carried out by a number of transducer manufacturers because of the expected potential reduction in fabrication cost.
The prior art includes several fabrication methods for multidimensional arrays including the electrical connection method for 2D arrays disclosed in U.S. Pat. No. 5,311,095 to Poulin et al. This method provides for a multi-layer ceramic connector as well as a mismatching layer for extending the electrodes of the array elements to the connection cables. The transducer is made from standard piezoelectric material and the method enables the transducer manufacturing process to be carried out by using a simple bonding operation. However, the thickness of the mismatching layer and the MLC favor the occurrence of reverberations in the pulse response of the transducer. Furthermore, the surface of each transducer element is so small that strong electrical reflections are induced which appear as oscillations in transducer response due to impedance mismatching.
One prior art reference which addresses the problems of impedance mismatch in linear arrays is U.S. Pat. No. 4,958,327 to Saito wherein a Nxc3x97layer piezoelectric member is used in linear transducer arrays. However, the technique for providing connections between the piezoelectric layers is limited to lateral short circuits and has provide to be inadequate for 2D transducer array fabrication.
A further technique is described in U.S. Pat. No. 5,381,385 to Greenstein et al which discloses a method for obtaining multilayer piezoelectric structures for use in matrix transducer arrays. Green ceramic layers are provided with holes performed in the surface thereof. Conductive vias are provided through these holes. Thus, when several layers of green ceramic are assembled together to form a multilayer structure, and the piezoelectric layers are connected in parallel, the capacitance is increased by a factor of the square of the number of layers, resulting in advantages which will be evident to one skilled in the art. However, this connection approach requires high volume fabrication in order to be cost effective as well as a high degree of precision in positioning the assembly of layers. Moreover, a via obtained by mechanical drilling or laser machining is subject to peripheral depoling of the surrounding material which may affect transducer performance and the homogeneity of the surface of the transducer.
In U.S. Pat. No. 5,548,564 to Smith et al, there is described a multilayer ceramic 2D array transducer wherein the piezoelectric layers are preferably assembled in parallel in an arrangement which is said to provide enhanced capacitance and impedance characteristics to the array. The method described can also be extended to composite material. However, as discussed, the feasibility of efficiently making such a transducer array depends on accurate alignment of the different layers of piezoelectric and further, the drilling of microholes remains an uncertain approach regarding reliability and will also affect the homogeneity of the array, and homogeneity is an essential criteria for acoustic devices.
U.S. Pat. No. 5,825,119 to Ossmann et al describes a multilayer structure-based transducer array for harmonic imaging wherein several operating modes are provided. The Ossmann et al patent is concerned with providing a transducing system having the ability to operate at two selected frequencies in order to transmit ultrasonic waves at one frequency and to receive the echoes from the structure under test at the other frequency. The method for driving the multilayer transducers is described in connection with embodiments including diode circuits or transistors or varistors or Zener diodes. This description is limited to double layer devices which are well adapted to harmonic imaging and there is no description of the use of this approach in 2D array constructions and the connection problem associated with such constructions has not been addressed.
In accordance with the present invention, methods are provided for manufacturing 1.5D and 2D ultrasonic array transducers which overcome the various drawbacks and disadvantages mentioned above. The use of piezoelectric multilayer structures as vibrators enables the resultant transducer to satisfy stringent requirements, including those concerning the capacitance characteristics and impedance homogeneity of the transducer. According to a first aspect of the invention, there is provided a novel method of making 1.5D transducer arrays wherein transducer configurations are produced which have shifted elevation apertures from the center to the edge of the array, and wherein the method produces uniformity of transducer capacitance independently of location in the elevation plane of the array. A further aspect of the invention concerns a method of connecting and using standard multilayer actuators to build high performance, cost effective 2D transducer arrays. The method enables manufacturing of superior 2D transducer arrays which possess electrical characteristics comparable to those of conventional linear arrays.
Generally speaking, the present invention employs multilayer piezoelectric fabrication technology to produce nD ultrasonic transducer arrays (e.g., 1.5D and 2D transducers) while avoiding the above-mentioned problems and limitations inherent in this type of device. Novel manufacturing and interconnection methods are provided which produce multi-dimensional transducer arrays useful in imaging or NDT applications. The methods of the invention simplify the manufacturing process and reduce the time necessary for fabrication of the transducer arrays. Moreover, reliability is greatly improved and new transducer configurations can be made as well.
According to another aspect of the invention, a 1.5D linear transducer array is provided in which impedance compensation is provided between the elemental transducer forming the array. Preferably, this impedance compensation is provided by varying the number of layers provided in each row of the elemental transducers so as to make feasible tuning of the impedance parameter.
Yet another aspect of the invention concerns the provision of a 2D transducer array or matrix transducer array wherein elemental transducers are produced by dicing a conventional multilayer actuator to form equal sized elements (pillars) which are embedded in a polymer matrix. Alignment of the individual pillars is provided by microholes which are performed on the surface of the multilayer actuators and which are removed with the dicing of the pillars.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.